Aircraft with electric or hybrid electric propulsion and aerodynamic braking implemented by the rotor of an electric machine and engine group for such an aircraft

Abstract

A drive unit (18), such as an intelligent motor, comprises an electronic control unit (20) and an electric machine (22). The electronic control unit is configured to selectively operate the electric machine in a propulsion mode and in an aerodynamic braking mode. In the latter mode, the electronic control unit is configured to control the torque T of the electric machine as a function of the rotational speed N of its rotor (22B) according to a polynomial control law, where "n" is a natural number greater than or equal to 2, and is a set of numerical coefficients, the coefficient of order n being strictly negative. The use of a polynomial law allows for stable operation and is not dependent on the reliability of sensors external to the drive unit, such as airspeed sensors on an aircraft. Figure for the abstract: Figure 1

Description

Title of the invention: Electric or hybrid electric aircraft with aerodynamic braking implemented by the rotor of an electric machine and engine unit for such an aircraft Technical field

[0001] The present invention relates to the general field of aircraft with electric or hybrid electric propulsion.

[0002] It relates more particularly to an optimization of a propulsion system of such an aircraft for the implementation of aerodynamic braking by a propeller in flight, which in certain embodiments can be coupled to an energy storage unit to recharge the latter and thus implement regenerative braking.

[0003] The invention is the result of technological research aimed at developing the use of electrical technologies to ensure the propulsion of aircraft, and thus contributes to reducing their environmental impact and the fight against climate change. State of the prior art

[0004] The principle of propelling aircraft by means of electric motors driving rotating propellers or fans, alone within electric propulsion systems, or combined with thermal engines within hybrid propulsion systems, is known.

[0005] The use of electric or hybrid propulsion means within aircraft offers numerous advantages, including the possibility of implementing regenerative braking in certain phases of flight. Such regenerative braking consists of using an electric motor of the aircraft in generator mode, that is to say, ensuring that the aerodynamic force applied to the propeller due to the aircraft moving forward in the air results in a torque applied to the rotor of the motor which is in the opposite direction to the direction of rotation of the rotor, so as to produce electrical energy and store the latter in a battery. In other words, it involves using the propulsion system in wind mode.

[0006] Such a possibility makes it possible in particular to increase the flight autonomy of aircraft and to improve the maneuverability of aircraft by reducing the duration of the descent, approach and landing phases thanks to the braking effect of the propulsion system. In cases where the maximum speed admissible by such an aircraft is limited by the mechanical characteristics of the propeller or the fan coupled to the electric motor (and not by other parts of the aircraft), reducing the rotational speed of the electric motor by means of regenerative braking also makes it possible to reduce the risk of propeller overspeed and thus increase the maximum speed permissible by the aircraft.

[0007] More generally, whether or not one is seeking to recharge an energy storage unit, the possibility of operating an electric motor of the aircraft in generator mode offers the possibility, by coupling an electrical load to it, of implementing aerodynamic braking of the aircraft by means of the propeller, which makes it possible to effectively reduce the speed of the aircraft during the approach and landing phases. Such aerodynamic braking is particularly advantageous in the case of small aircraft with a relatively high finesse, making it difficult to slow down the aircraft during the approach phase.

[0008] The implementation of such aerodynamic braking, whether or not accompanied by the recharging of an energy storage unit, nevertheless poses difficulties in controlling the electric motor in a manner allowing stable operation of the propulsion system and therefore of the aircraft. In addition, it is desirable that the piloting of the aircraft, in particular the control of the propulsive power or the intensity of the aerodynamic braking, depending on the flight phases, is done in a simple and intuitive manner for the pilots. Statement of the invention

[0009] The invention aims in particular to enable aerodynamic braking implemented by the rotor of an electric machine, allowing stable and safe operation.

[0010] The invention proposes for this purpose an engine group for aircraft, comprising an electric machine and an electronic control unit, the electric machine comprising a stator and a rotor, the electronic control unit being configured to selectively operate the engine group in: • a propulsion mode, in which the electronic control unit supplies the electric machine with electrical energy and controls the latter to operate as a motor, so as to drive the rotor into rotation; and • an aerodynamic braking mode, in which the electronic control unit controls the electric machine to operate as a generator so as to generate electrical energy under the effect of rotation of the rotor.

[0011] In the aerodynamic braking mode, the electronic control unit is configured to control the torque T of the electric machine as a function of the rotational speed N of the rotor according to a polynomial control law T(N) = Ao + A] x N+ ... A„x N", "n" being a natural integer greater than or equal to 2, and {Ao ... Ars] being a set of numerical coefficients, the coefficient A„ of order n being strictly negative.

[0012] The use of a polynomial law amounts to controlling the electric machine so that it behaves substantially like a fan, which allows particularly stable operation. In addition, the control of the electric machine is thus not dependent on information provided by sensors independent of the engine group such as air speed sensors of the aircraft, and is therefore not dependent on the reliability of such sensors.

[0013] The present invention thus proposes a safe, simple, stable, robust and easily implementable control principle.

[0014] In certain embodiments, the control unit and the electrical machine are integrated, for example within the same casing, so as to form an autonomous assembly of the type commonly referred to as a “smart motor”.

[0015] In preferred embodiments, the integer n is equal to 2.

[0016] In preferred embodiments, the coefficient Ao is strictly positive.

[0017] Such a positive coefficient Ao makes it possible to ensure that from a minimum threshold of rotor rotation speed, the torque T becomes driving again so that the motor unit returns to propulsion mode and thus prevents the rotor speed from falling below this minimum threshold.

[0018] Such an arrangement is particularly advantageous in cases where the electrical machine is a machine controlled without a speed sensor from an estimator, as explained for example in document FR2954020A1. Indeed, such a speed estimator requires that the rotation speed of the rotor remains above a certain threshold, so as not to risk a loss of control of the machine.

[0019] In preferred embodiments, the electronic control unit is configured to assign different sets of values ​​to the set of coefficients {Ao... An} depending on an aerodynamic braking setpoint level.

[0020] In preferred embodiments, the electronic control unit is configured such that at least some of the sets of values ​​attributable to the set of coefficients { ... An} define control laws corresponding respec tively to curves which, on a diagram representing the torque T as a function of the rotational speed N, cover a continuous region of control laws intended to be implemented in response to aerodynamic braking instructions, going up to a curve corresponding to a “FULL” instruction of maximum aerodynamic braking.

[0021] In preferred embodiments, the electronic control unit is configured such that another of the sets of values ​​attributable to the set of coefficients ficients {. A„} defines a control law, intended to be implemented in response to a “SLOW” instruction for maximum slowing down of the propeller, and corresponding to a curve located outside said region and separated from the latter by a region in which respective minima of propeller curves are located.

[0022] The invention also relates to an aircraft, comprising a power unit of the type described above, a propeller coupled to the rotor of the electric machine, and an electric load configured to store or dissipate the electrical energy generated by the electric machine in the aerodynamic braking mode.

[0023] In the present disclosure, the term “propeller” is used to designate indifferently a propeller or a fan.

[0024] In preferred embodiments, the aircraft comprises an energy storage unit configured to power the electric machine via the electronic control unit in the propulsive mode, and constituting said electrical load, whereby the aerodynamic braking mode is a regenerative braking mode in which the power unit recharges the energy storage unit from the energy generated by the electric machine.

[0025] In preferred embodiments, the aircraft comprises a thrust lever configured to selectively provide a thrust setpoint or an aerodynamic braking setpoint to the electronic control unit, the latter being configured to operate the engine group in the propulsion mode in response to said thrust setpoint, and to operate the engine group in the aerodynamic braking mode by controlling the torque T of the electric machine according to said polynomial control law, in response to said aerodynamic braking setpoint.

[0026] In preferred embodiments, the electronic control unit is configured to modify all or part of the coefficients {. An} of the polynomial control law in response to a modulation of a level of the aerodynamic braking setpoint caused by a movement of the thrust lever within an adjustment zone.

[0027] In preferred embodiments, the adjustment zone comprises a “REV” range of progressive increase in the level of aerodynamic braking, going from a “0” position corresponding to a zero braking setpoint, to a “FULL” position corresponding to said “FULL” setpoint of maximum aerodynamic braking, and a “SLOW” position corresponding to said “SLOW” setpoint of maximum slowing of the propeller.

[0028] The invention also relates to a method for operating an aircraft of the type described above, comprising: • providing a thrust command to the electronic control unit, whereby the electronic control unit supplies the electric machine with electrical energy and controls the latter to operate as a motor, so as to drive the rotor and therefore the propeller in rotation; and • providing an aerodynamic braking instruction to the electronic control unit, whereby the latter controls the electric machine, according to said polynomial control law, to operate as a generator so as to generate electrical energy under the effect of rotation of the rotor driven by the propeller.

[0029] The invention also relates to a computer program product comprising code instructions for executing the steps of the method defined above, when said program is executed on an electronic control unit. Brief description of the drawings

[0030] The invention will be better understood, and other details, advantages and characteristics thereof will appear on reading the following description given by way of non-limiting example and with reference to the appended drawings in which:

[0031] [Fig-1] is a schematic view of an electrically powered aircraft according to a mode preferred embodiment of the invention;

[0032] [Fig.2] is a diagram illustrating curves representing the torque T as a function of the rotational speed N of a propeller of the aircraft of [Fig.l], in stabilized mode respectively for different conventional speeds of the aircraft;

[0033] [Fig.3] is a diagram similar to [Fig.2], further illustrating curves representing control laws of the regenerative torque T as a function of the rotation speed N of the propeller, corresponding respectively to different operating instructions;

[0034] [Fig.4] is a schematic top view of a thrust lever of the aircraft of [Fig.l], Detailed disclosure of preferred embodiments

[0035] [Fig.l] very schematically illustrates an electrically powered aircraft 10, the propulsion system 12 of which comprises an electrical load 14, an electrical power supply bus 16, at least one motor group 18, and a propeller 24. The electrical load 14 is, in the preferred example illustrated, an energy storage unit, typically comprising at least one electrochemical battery.

[0036] The motor group 18 comprises an electronic control unit 20 and an electric machine 22. The electronic control unit 20 typically comprises an electric converter. The electric machine 22 comprises a stator 22A, and a rotor 22B coupled by an output shaft to propeller 24.

[0037] In the illustrated example, the propeller 24 is a fixed-pitch propeller.

[0038] The energy storage unit 14 is connected to the electric machine 22 via the electrical power supply bus 16 and the electronic control unit 20 so that, in a propulsive operating mode, the energy storage unit 14 supplies the electric machine 22 via the electronic control unit 20 so that the electric machine 22, then operating as a motor, drives the propeller 24 in rotation, while, in a regenerative operating mode, the energy storage unit 14 is recharged by an electric current produced by the electric machine 22, then operating as a generator, under the effect of a drive of the rotor 22B by the propeller 24. This latter operating mode aims to allow regenerative braking in flight and thus generally offer the advantages set out above.

[0039] For this purpose, the converter of the electronic control unit 20 is configured to operate as an inverter in the propulsion operating mode, so as to convert a direct current DC, coming from the energy storage unit 14 and conveyed by the electrical power supply bus 16, into an alternating current AC suitable for powering the electrical machine 22, typically a three-phase current.

[0040] Furthermore, the converter of the electronic control unit 20 is configured to operate as a rectifier in the regenerative operating mode, so as to convert an alternating current AC produced by the electrical machine 22 operating as a generator, into a direct current DC suitable for recharging the energy storage unit 14.

[0041] The motor group 18 comprises, for example, a casing within which the constituents of the group, including the electronic control unit 20 and the electric machine 22, are brought together so as to form an assembly commonly called a “smart motor” or “intelligent motor”.

[0042] Although the present detailed description relates to an example in which the energy supplied by the electrical machine 22 operating as a generator is collected (in the energy storage unit 14), the invention is also applicable in cases where the electrical load is configured to dissipate this energy without storing it. In such a case, the corresponding operating mode is not a regenerative braking mode but, more generally, an aerodynamic braking mode.

[0043] A specific feature of the present invention lies in a law for controlling the electric machine 22 by the electronic control unit 20 in the regenerative braking phases, or more generally aerodynamic braking, in this case a law for controlling the resistive torque developed by the electric machine to generate electrical energy by opposing the rotation of the propeller 24.

[0044] To facilitate understanding of the following, the diagram in [Fig. 2] shows propeller curves C1-C4 of the propulsion system 12, i.e. curves of the torque T (in Newton-meters Nm) as a function of the rotational speed N (in revolutions per minute RPM), respectively for different conventional speeds (in knots kn) of the aircraft. By "conventional speed", also called KCAS for "Knot Calibrated Air Speed", is meant the indicated speed of the aircraft, corrected for position and instrument errors, and therefore equal to the true speed in standard atmospheric conditions at sea level. It should be noted that this diagram only shows the portion of the propeller curves corresponding to generator operation (negative torque T and positive speed N), and therefore omits the portion corresponding to propulsive operation (positive torque T and positive speed N).

[0045] On each of the curves C1-C4, the respective point P1-P4 located at the point where the curve intercepts the abscissa axis (zero torque T) corresponds to a speed stabilization point when no driving or braking force is applied to the propeller shaft 24.

[0046] Beyond points P1-P4, each of the curves C1-C4 shows what value of torque T makes it possible to stabilize the speed N of the propeller as a function of the value of said speed N, for a given conventional speed of the aircraft. Generally speaking, for a given speed N, a torque T greater than that given by the curve tends to induce an acceleration of the propeller 24 while a torque T less than that given by the curve tends to induce a slowing down of the propeller. This is explained by the fact that increasing the torque T amounts to reducing the resistance to rotation of the propeller 24 induced by the electrical machine 22, that is to say reducing the resistive torque, while reducing the torque T amounts to increasing this resistance.

[0047] Each helix curve C1-C4 has a respective minimum M1-M4 determining the minimum torque Tlmin-T4min (i.e. the maximum resistive torque) applicable to the helix and the corresponding speed Nlmin-N4min. In practice, taking into account the concave shape of each helix curve around its minimum M1-M4, the maximum power that can be generated (dissipated or collected) during braking for a given helix curve is obtained at a speed slightly higher than the speed Nlmin-N4 min for which the torque T is minimal.

[0048] For each curve, it is remarkable that for speeds N lower than the speed corresponding to the minimum M1-M4 considered, the system remains naturally unstable. Indeed, with a simple control law which would consist of adjusting the torque T to satisfy a speed setpoint N, it appears that in the speed zone close to the minimum M1-M4 considered, there would be no preferred direction allowing the speed to be maintained: an increase in the torque T would randomly lead to either an increase or a reduction in the speed N. Moreover, in the zone of speeds N lower than the speed corresponding to the minimum M1-M4 considered, an increase in the torque T (i.e. a reduction in the resistive torque) from a given operating point P, normally intended to accelerate the propeller, will in reality have the effect of slowing down the propeller by following the curve C1-C4 considered in the direction of decreasing speeds N.

[0049] Another conceivable control law could consist of controlling the electric motor 22 as a function of a torque setpoint T. However, such a control mode presents the risk that a setpoint corresponding to too high a resistive torque (i.e. of too high an absolute value), taking into account the conventional speed of the aircraft, could cause the propeller to stop, and thereby compromise control of the motor and therefore of the aircraft.

[0050] The present invention proposes a simple, stable, robust and easily implementable control principle, which will now be explained.

[0051] This principle consists, in general, in configuring the electronic control unit 20 so as to control the electrical machine 22 according to a polynomial control law of the torque T as a function of the rotation speed N, that is to say such that T(N) = Aq + Af x N + ... An x N”, “n” being a natural integer greater than or equal to 2, and the coefficient An (of order n) being strictly negative (since, by convention, a positive torque T corresponds to a motor torque while a negative torque T corresponds to a resistive torque).

[0052] The coefficients {Aq... A„} are digital coefficients which may be fixed. However, in the preferred example illustrated, the electronic control unit 20 is configured to assign different sets of values ​​to the set of coefficients {Ao ... A„} depending on a level of an aerodynamic braking setpoint, for example regenerative braking, as will become more clear in the following. In addition, the coefficients {Ao ... A„} are preferably defined by respective continuous functions of the braking setpoint level in a determined braking setpoint range.

[0053] In the preferred example described, the law is of order 2, that is to say such that T(N) = +A] x N +A, x N2-

[0054] The use of a polynomial law amounts to controlling the electrical machine 22 so that it behaves substantially like a fan, which allows stable operation.

[0055] The inventors have in fact noticed that the helix curve visible in [Fig.2] has characteristics similar to those of an operating curve of an asynchronous motor, with in particular: - at zero speed, a starting torque; - a maximum torque point; - beyond the maximum torque point, a drop in torque in the high speed region, resulting in zero torque.

[0056] However, asynchronous machines have stable operating speeds when they are associated with fan-type loads, the latter behaving according to polynomial laws.

[0057] Figure 3 is a diagram similar to Figure 2 but additionally showing L1-L4 curves of the torque T as a function of the speed N according to the second-order polynomial law specified above, respectively for different sets of values ​​of the set of coefficients { Aq... An} •

[0058] Curve L1 delimits a continuous region R of control laws which are defined by the coefficients [Ao... An} determined by the aforementioned continuous functions when the braking setpoint level sweeps the determined braking setpoint range, and which are intended to be implemented in response to aerodynamic braking setpoints, in this case regeneration setpoints.

[0059] In particular, the curve L1 defines a control law intended to be implemented in response to a maximum braking instruction, called “FULL” in the following. The curve L1 intercepts the propeller curves at points slightly offset to the right relative to the minima M1-M4 and therefore close to the points at which the power generated is maximum. On this curve L1, the propeller 24 thus operates with high efficiency in wind mode, which makes it possible to maximize aerodynamic braking and, in the illustrated embodiment, to maximize the recharging of the energy storage unit 14.

[0060] The region R is thus defined above the curve Ll in [Fig.3].

[0061] Curves L2 and L3 are two examples of control laws corresponding to regeneration instructions less intense than the maximum regeneration defined by the LL curve The two curves L2 and L3 are thus in the R region. The determined braking instruction range, on which the aforementioned continuous functions operate which make it possible to define the coefficients {Ao ... A„} as a function of the braking instruction level, typically extends from a zero braking instruction level up to the maximum braking instruction level “FULL”.

[0062] Curve L4 defines a control law intended to be implemented in response to a maximum slowdown instruction of the propeller 24, called “SLOW” in the following. This control law is defined so as to implement aerodynamic or regenerative braking which reduces the speed of the propeller 24 as much as possible without risking the electric motor 22 stopping. The SLOW instruction is of interest in the context of a multi-engine aircraft having a fixed-pitch propeller capable of being implemented flag, or an aerobatic aircraft, or a vertical take-off aircraft for which it may be necessary to reduce propeller speed to a minimum in the event of the loss of an opposing engine.

[0063] The curve L4 is located outside the region R and separated from it by a region MIN in which the respective minima of the curves of the propeller 24 are located, for example the minima M1-M4 of the curves C1-C4. Preferably, no control law is defined in the region MIN, so as to avoid certain braking level setpoints corresponding to two possible control laws on either side of a corresponding minimum Mx (one in the region R and the other in the region MIN), which would complicate control from the point of view of a pilot.

[0064] For each of the control laws thus defined, the coefficient A) is advantageously strictly positive, which results in a positive torque T (i.e. a motor torque) at the lowest rotation speeds N of the propeller, tending to re-accelerate the propeller 24 and thus making it possible to reduce the risks of uncontrolled stopping of the engine. The coefficient A) is for example the same for the different control laws. The transition from one law to another within the region R, or between the region R and the curve L4, is thus carried out by changing all or part of the coefficients A and A2.

[0065] Figure 4 illustrates a thrust lever 30 intended to allow a pilot of the aircraft to control the thrust of the engine group 18. More specifically, the lever 30 is movable in rotation according to an angular travel for example delimited by an opening 32 provided in a housing 34, continuously in an adjustment zone 36, on the front side, corresponding to a range of positive thrust setpoints for propelling the aircraft, and in an adjustment zone 38, on the rear side, corresponding to a range of aerodynamic braking setpoints, in this case regenerative braking, passing through an adjustment notch “0” corresponding to a zero propulsion setpoint without aerodynamic braking, which may correspond to freewheel operation, to operation under a zero torque setpoint, or even to a setpoint comprising a positive torque heel in the manner of the positive heel defined by the coefficient A) as explained above.

[0066] A movement of the thrust lever 30 in the adjustment zone 38 makes it possible to modulate a level of the aerodynamic braking setpoint and thus cause a modification of all or part of the coefficients {A) • ■ • A} of the control law implemented by the electronic control unit 20.

[0067] More precisely, the adjustment zone 38 comprises a “REV” range of progressive increase in the level of aerodynamic braking, going from the “0” position to a “FULL” position corresponding to the aforementioned “FULL” braking instruction, followed by a position or notch corresponding to the “SLOW” instruction aimed at slowing down the propeller 24 as much as possible without risking stopping the electric motor 22.

[0068] It must therefore be understood that when the thrust lever 30 is positioned in its “REV” range or in the “FULL” position at the limit of the “REV” range, the electrical machine 22 is controlled according to a control law belonging to the region R of [Fig.3], or, where appropriate, corresponding to the “FULL” setting associated with the curve L1 of [Fig.3].

[0069] Similarly, when the thrust lever 30 is positioned on the “SLOW” notch, the electric machine 22 is controlled according to a control law corresponding to the curve L4 of [Fig.3].

[0070] In the example illustrated, the movement of the thrust lever 30 in the adjustment zone 38 (i.e. below the notch “0”) therefore has the effect of modifying all or part of the coefficients Aj and A2.

[0071] Generally speaking, the thrust lever 30 thus allows simple and intuitive control of the propulsion system 12, similar to a conventional thrust control from the pilots' point of view.

[0072] In the example described, the torque T of the electric motor is determined by the converter of the electronic control unit 20. The manner in which the converter implements the control law is however not the subject of the present disclosure. The operation of the converter is based for example on conventional control techniques, such as vector control techniques. One way of configuring the converter 20 to control the torque T can be found in the document EPI 138539B1.

[0073] Furthermore, the electrical machine 22 may be of the type controlled without a speed sensor, from an estimator, as explained for example in the document FR2954020. As explained above, such a speed estimator requires that the rotation speed of the rotor 22B remains above a certain threshold, otherwise control of the machine risks being lost.

[0074] The coefficient A) is preferably a strictly positive number chosen to ensure that the rotation speed of the rotor 22B remains above this minimum threshold and thus avoid a loss of control of the electrical machine 22.

[0075] In such a case, the “SLOW” position plays a safety adjustment role insofar as such an adjustment offers a minimum speed allowing the pilot to considerably reduce the speed of the propeller 24 while avoiding an uncontrolled shutdown of the electric machine 22, which would result in particular in a loss of the rotor speed estimate.

Claims

Claims

1. An aircraft engine unit (18), comprising an electric machine (22) and an electronic control unit (20), the electric machine (22) comprising a stator (22A) and a rotor (22B), the electronic control unit (20) being configured to selectively operate the engine unit (18) in: • a propulsion mode, in which the electronic control unit (20) supplies the electric machine (22) with electrical energy and controls the latter to operate as a motor, so as to drive the rotor (22B) in rotation; and • an aerodynamic braking mode, in which the electronic control unit (20) controls the electric machine (22) to operate as a generator so as to generate electrical energy under the effect of a rotation of the rotor (22B);characterized in that, in the aerodynamic braking mode, the electronic control unit (20) is configured to control the torque T of the electric machine (22) as a function of the rotational speed N of the rotor (22B) according to a polynomial control law T(N) = Ao + Aj XN + ... An x N", "n" being a natural integer greater than or equal to 2, and {Ao... An} being a set of numerical coefficients, the coefficient An of order n being strictly negative.;

2.

3. Engine group according to claim 1, in which n is equal to 2. Engine group according to claim 1 or 2, in which the coefficient Ao is strictly positive.

4. An engine unit according to any one of claims 1 to 3, wherein the electronic control unit (20) is configured to assign different sets of values ​​to the set of coefficients {Ao ... A„} depending on an aerodynamic braking setpoint level.

5. Engine group according to claim 4, wherein the electronic control unit (20) is configured so that at least some of the sets of values ​​attributable to the set of coefficients {Ao ... Aw} define control laws corresponding respec- tively to curves (Ll, L2, L3) which, on a diagram representing the torque T as a function of the rotation speed N, cover a continuous region (R) of control laws, intended to be implemented in response to aerodynamic braking instructions, going up to a curve (Ll) corresponding to a “FULL” instruction of maximum aerodynamic braking.

6. Engine unit according to claim 5, in which the electronic control unit (20) is configured so that another of the sets of values ​​attributable to the set of coefficients defines a control law, intended to be implemented in response to a "SLOW" instruction for maximum slowing of the propeller (24), and corresponding to a curve (L4) located outside said region (R) and separated from the latter by a region (MIN) in which respective minima (M1-M4) of curves (C1-C4) of the propeller (24) are located.

7. An aircraft, comprising a power unit (18) according to any one of claims 1 to 6, a propeller (24) coupled to the rotor (22B) of the electric machine (22), and an electrical load (14) configured to store or dissipate electrical energy generated by the electric machine (22) in the aerodynamic braking mode.

8. An aircraft according to claim 7, comprising an energy storage unit configured to power the electric machine (22) via the electronic control unit (20) in the propulsive mode, and constituting said electrical load (14), whereby the aerodynamic braking mode is a regenerative braking mode in which the power unit (18) recharges the energy storage unit from the energy generated by the electric machine (22).

9. Aircraft according to claim 7 or 8, comprising a thrust lever (30) configured to selectively provide a thrust setpoint or an aerodynamic braking setpoint to the electronic control unit (20), the latter being configured to operate the engine group (18) in the propulsive mode in response to said thrust setpoint, and to operate the engine group (18) in the aerodynamic braking mode by controlling the torque T of the electric machine (22) according to said polynomial control law, in response to said aerodynamic braking setpoint.

10. An aircraft according to claim 9, wherein the power unit (18) is a power unit according to claim 4 or 5, and the electronic control unit (20) is configured to modify all or part of the coefficients- ficients { Ao... An ) of the control law in response to a modulation of a level of the aerodynamic braking setpoint caused by a movement of the thrust lever (30) within an adjustment zone (38).

11. Aircraft according to claim 10, in which the engine group (18) is an engine group according to claim 6, and in which the adjustment zone (38) comprises: • a “REV” range of progressive increase in the level of aerodynamic braking, going from a “0” position corresponding to a zero braking setpoint to a “FULL” position corresponding to said “FULL” setpoint of maximum aerodynamic braking, and • a “SLOW” position corresponding to said “SLOW” setpoint of maximum slowing of the propeller (24).

12. A method of operating an aircraft according to any one of claims 7 to 11, comprising: • providing a thrust setpoint to the electronic control unit (20), whereby the electronic control unit (20) supplies the electric machine (22) with electrical energy and controls the latter to operate as a motor, so as to drive the rotor (22B) and therefore the propeller (24) in rotation; and • providing an aerodynamic braking setpoint to the electronic control unit (20), whereby the electronic control unit (20) controls the electric machine (22), according to said polynomial control law, to operate as a generator so as to generate electrical energy under the effect of a rotation of the rotor (22B) driven by the propeller (24).

13. A computer program product comprising code instructions for executing the steps of the method according to claim 12, when said program is executed on an electronic control unit (20).

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